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Old 25th January 2004, 05:11 AM   #11
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I have been in touch with Derek for a few years. I have a pair of Tango XE20s3.5 and a pair of Tamura 10 H chokes. I am going to build this simple amplifier. I just moved again and I now have a dedicated spare bedroom for my vintage and new audio projects. For the power transformers I will prolly use the suggested universal power ones from Angela at $99 each. I have a few GZ37 rectifiers already in my tube stash. The mesh plate 300b tubes are getting rave reviews so for under 300 a pair they will be my 300b tube of choice. What speakers will you be using with these? To me the most important part of any stereo system are the speakers. My speakers are 93dB efficient and Derek told me his speakers are around 87dB efficient and his amp works with those speakers. The only other things I need are the chassis, and tube sockets, which will prolly be the wood ones from Wellborne with brass top plates. These are simple enough to build that they are worth a try even for me. I have a flex drill that will work to cut the chassis on my small wooden bench. If I screw up I will just pay a local metal shop to cut the top plates for me. I think punches seem to leave extra marks on the chassis. There is another project that Gordon Rankin gave for free to DIY the audio community called the Rankin 2A3, A.K.A. "The Baby Onganku." If your speakers only need 3 to 4 wpc these look really interesting. There is a complete suggested layout for that too with the plans in PDF format. I simply blew up the images to make them easeir to read for me. I would not use the silver transformers, just the regular Magnequest iron. I am also looking into "James" iron that is supposed to be very good sounding and quite reasonably priced. I am thinking of these for my second project for my Dad's Tad TD-4001 drivers. All they need is a few watts and the Rankin 2a3 should work well with them. I could not think of two easier first time projects that either Derek's version of the 300b, or the Rankin 2A3 mono blocks. Just keep a sense of humor and remember audio is supposed to be fun. If you are thinking of building these and want to save some dollars I have some info on a few other output transformers, and a new Tamura made c-core power transformer available at under $100. Email me and I will look for the links for you. It looks interesting but I want to use my iron. It took me long enough to save and scrounge for it.. Maybe the inexpensive iron for the 2a3 project just for fun is worth a try. Grab your soldering iron and have some fun. The only thing left for me to decide is whether or not to change the power supply to try to reduce the 5mv of hum. Again with my 93dB efficient horn speakers that would not be a problem for me anyway.
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Old 25th January 2004, 11:22 PM   #12
DVDHack is offline DVDHack  Australia
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After spending some time researching the various designs it would appear that the SJS Amp below meets many of the improvements recommended here.

http://www.users.globalnet.co.uk/~va...s/SE108IT2.GIF

The amp employs no feedback, uses an interstage driver TFR and a single voltage amplifier stage. The only compromise appears to be a DC heater on the 300B but I may consider an AC supply initially and update it if its too noisy (can I use the supply on the GZ37 or does it need to be a separate winding?)

What are the thoughts around this design?

Lot of Iron in this design - think I'll need a hand lifting this if I build it.


Regards

Ralf
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Old 26th January 2004, 10:29 AM   #13
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Konnichiwa,

Quote:
Originally posted by DVDHack
After spending some time researching the various designs it would appear that the SJS Amp below meets many of the improvements recommended here.
Yes, this is a really good one, the limiting factor is the IT, I have yet to find a SE IT I find good enough in terms of resolution and bandwidth though.

Quote:
Originally posted by DVDHack
The only compromise appears to be a DC heater on the 300B
This is not a compromise but a necessity. The trick is to get DC to sound good. I recommend changing the DC Heatersupply to CLC with an added common mode filter, as shown in the schematic I have referenced further up.

Quote:
Originally posted by DVDHack
(can I use the supply on the GZ37 or does it need to be a separate winding?)
The Rectifier heater winding is connected to +B so using it to power the 300B heater would be quite disasterous. PLEASE read up on Valve theory before making ANY changes on established, tested designs, however innocent these changes may appear to be.

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Old 26th January 2004, 12:04 PM   #14
Mike C is offline Mike C  United Kingdom
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Quote:
Originally posted by DVDHack
After spending some time researching the various designs it would appear that the SJS Amp below meets many of the improvements recommended here.

Regards

Ralf

Yes, it's a very good design; but rather than just tell you this, we should show you why and help you work it out for yourself.

Separate PSUs for driver stage and output stage are good for dynamics and clarity. I use this in my 'Valentine' 300B amp, right down to using separate mains TXs (supplied by Steve Shilton funnily enough).
I guess that with a shared PSU, interactions between the stages under load will give a crosstalk effect, possibly lowering dynamics due to sag of the PSU. Also, I guess the crosstalk may be a touch out of phase, giving an interference effect. OK, the effects are slight; but we're all looking for 'no compromise' designs (even if we never get there!)

The design uses 'fixed' bias, i.e. a negative voltage applied to the grid of the 300B. A very good method, often reckoned to be 'best'.
The main alternative is cathode bias (as you'll have seen in most designs) ... this generally needs a cathode bypass cap and because it has to be large, it is almost always an electrolytic cap (and these are usually of marginal quality).
Fixed bias eliminates the need for this cap.
Note a funny thing; the bias is adjustable by a pot, but it's still called 'fixed' as it's not cathode bias .... no, I don't understand why either.

Note the elegant way the interstage transformer does two things.
It applies the bias voltage to the 300B grid; and it couples the signal without needing a coupling cap.

To appreciate the design further, it's a very good idea to look up the curves for the driver valve; see if you can see why that valve was chosen. Don't be afraid to consider others or ask for suggestions; but I'm confident Steve S has made a good choice.

Think about the overall design. An issue is that we want (IMHO) to be able to drive the 300B grid by about + and - 150V (!!). Yes, I know that's 2 x the bias point ... but people say it gives a good result and my highly limited experience confirms this.
Now at first sight, 190V B+ on the driver stage looks like it won't do it? That's my only query with the design.
However, I guess that dynamic effects due to the inductance of the IT means that it's OK; I'd love an expert to comment on this!

PSUs look good but one could consider smaller caps in some places (allowing use of film type) and one of the PSUs might be better with two stages rather than a single choke; but then again it might not!
And these are small points anyway.

For ITs, I'd suggest you look into Lundahl?

I like this design a lot and am considering the same schematic for my next power amp ....
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Old 26th January 2004, 03:39 PM   #15
protos is offline protos  Greece
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SJS looks good.Hey TL and others how about posting existing links to designs that you think exemplify the best out there.
How do you critique this design ?http://www.plitron.com/PDF/GLASSkoby.pdf
What I gather general agreement up to now is
1.Separate PS for driver
2.Fixed bias
3.AC heating
4.Valve rectification
5.Best OTX you can afford(Plitron,Tamura etc)
6.Useful to get power tx with heater windings and fixed bias windings.
I think the jury is still out on one or two driver stages or IT tx.
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Old 26th January 2004, 03:56 PM   #16
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Default Good points

"Separate PSUs for driver stage and output stage are good for dynamics and clarity. I use this in my 'Valentine' 300B amp, right down to using separate mains TXs (supplied by Steve Shilton funnily enough).
I guess that with a shared PSU, interactions between the stages under load will give a crosstalk effect, possibly lowering dynamics due to sag of the PSU. Also, I guess the crosstalk may be a touch out of phase, giving an interference effect. OK, the effects are slight; but we're all looking for 'no compromise' designs (even if we never get there!)"


DVD, I wrote an article several years ago addressing the problem of feedback through the power supply, as Mike explains above.

I agree with Wang about IT use about FR etc, but also from a distortion standpoint.

At high signal levels, transformers produce distortions, which when combined with the previous tube and output tube creates higher orders of distortions (I haven't even included the OPT).

For example, combining the 3rd harmonic from the first tube with the 2nd from the IT, and the 3rd from the output tube already produces the 18th harmonic, although fairly small. 2nd, 2nd, 2nd, produces 8th. 3rd, 2nd, 2nd produces 12th.

The amp actually is a combo of fixed and cathode bias, with a hum balancing pot.

Cheers,
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Old 26th January 2004, 06:38 PM   #17
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Konnichiwa,

Quote:
Originally posted by protos
Hey TL and others how about posting existing links to designs that you think exemplify the best out there.
Currently I am not really aware of a truely "no-compromise" design. Closest would be a combination of the SJS & Hoekstra Axiom Design, with two LCLC Supply filter sections.

I am just "specing out" such a design for myself. Intended is the following basic outline:

Medium MU/High GM Triode like WE417A/437A, russian 6S45/6S4, german EC8010/8020 or triode wired High Transconductance Valves with an Anode load choke (50% Mu Metal laminations), ultrapath cathode decoupling with variable cathode resistor and easily changed Valve mounting plates to allow a wide range of valves to be acopmmodated, operating point probably around 10-12mA.

The +B will be switchable around 350V, 280V & 250V for 250/180/150V anode voltage with up to 100V Bias for the Output Valve. PSU will be valve rectified and LCLC using 2-section chokes trimmed for minimum parasitic capacitance within each section and maximum leakage inductance between section to serve as combined differential & common mode chokes, PSU entierley using Film Capacitors with silver mica bypass..

Output Stage direct coupled with the anode current of the driver Valve providing the output stage Bias due to the voltage drop in the drivers anode and using a 5K nominal Output transformer (but with a secondary similar to the Tango X20S, but with an improved secondary winding arrangement set to minimise the unconnected sections of the secondary).

The +B Supply will be adjustable to 450, 400, 350, 300 & 250V to allow a range of Output Valves apart from the 300B to work as well as any of the "super 300B" Valves that work at higfher Voltage & current, again dual section Chokes as above and only Film Capacitors in the Supply and the usual .

The output valve heaters will be selectable 2.5V or 2.5V/5V/7.5V DC with the DC schottky rectified and CLCLC (only electrolytic capacitors in the Amp) smoothed. The two seperate mains transformers will be made with "balanced" primaries and multiple electrostatic screens to maximise the common mode rejection and with a "low flux" design and a small airgap to elimiate susceptibility to DC and potential saturation or high distortion of the core. In both transformers a conventional rectifier heater windiong is used to damp any potential rininging in the transformer/rectifier circuit. The driverstage +B path will include a suitable relais as interlock to switch on the output Mains transformer once the driverstage is operational and the bias voltage is present.

Looks like all Iron for this project will have to be scratch developed ith each item specifically tailored to the job (like first choke as swining input choke, second choke as super wideband filter choke etc.), including a silver secondary and possibly primary on the output transformer and possibly silver winding on the driver's anode load choke.

Quote:
Originally posted by protos
How do you critique this design ?http://www.plitron.com/PDF/GLASSkoby.pdf
Completely traditonal and not very interesting. Neither Mu-Follower nor White Follower (excellent measurements nonwithstanding) sound all that good and having used active regulation I know how hard it is to get good sound from it.

IF I where to recommend a DIY Design right now it would be a modified Jack Elliano DRD Amplifier from VTV, especially with a more relaxed 300B operating point, a PSU free from electrolytic capacitors and valve rectification. Or build the Marzio/Jelasi direct coupled Amplifier from Sound practices.

Quote:
Originally posted by protos
What I gather general agreement up to now is
1.Separate PS for driver
Not neccesarily, but surely helps. I found a RCRC filter from the main supply often sufficient occasionally even a CR filter, assuming the timconstant is high enough.

Quote:
Originally posted by protos
2.Fixed bias
Often more trouble than it's worth. Fixed bias rarely sounds as good as well implemented self bias scheme due to the clipping behaviour. The advantage of fixed bias is the elimination of the cathode bypass capcitor. If fixed bias is used it is VERY ESSENTIAL to minimise the grid circuits (DC) resistance which increases the demands on the driver or that failing increases the overload recovery of the circuit.

Quote:
Originally posted by protos
3.AC heating
Usually completely unacceptable for 5V and higher voltage valves due to excessive Hum and the IMD produced "veil" across the music. DC on the other hand can sound clinical and unpleasant. Solutions are various and will not be detailed here, look at several of muy designs to see what I found to work well.

Quote:
Originally posted by protos
4.Valve rectification
Absolutely. At least unil High Voltage schottky diodes that glow in the dark become cheaper than valve rectifiers.

Quote:
Originally posted by protos
5.Best OTX you can afford(Plitron,Tamura etc)
Yes. However, what constitutes the "best" output transformer and what ingrediences are needed is a matter of taste or so it seems.

Quote:
Originally posted by protos
6.Useful to get power tx with heater windings and fixed bias windings.
Probably. Mains Transformer design is another often overlooked science. Most commonly found mains transformers are not well suited to real world operation neccesitating extensive (and expensive) external power conditioning to allow good sound. The suceptibility of a given piece of gear to mains cable changes and mains conditioners as well as the "Midnight Hour" phenomenae are strong indicators for ****poor powertransformer design. Of course, a well designed power transformer is larger, more complex to make and subsequently more expensive than an excellent SE Output transformer.

Quote:
Originally posted by protos
I think the jury is still out on one or two driver stages or IT tx.
Two driver stages invariably lead to a much increased level of higher harmonics and related IMD in the signal input to the Output Valve. It is no problem with modern linelevel sources to use only two stages. With IT's I find that those which require to cope with DC bias tend to be too copmpromised in many areas to perform as intended. At low frequencies they run out of inductance and at high frequencies the large size makes winding geomertries that avoid resonances and early HF rolloff difficult to implement.

If true IT coupling is desired (as opposed to Parafeed IT or actually in essence LCL coupling) using differential driverstages allow interstage transformers with exceptional performance to made. Using a 1:1+1 input transformer, a 5687 or E182CC and a suitable high nickel content core PP Interstage 1CT:1+1 makes for an exceptional driver for either SE or PP Amplifiers, with superb level handling, squarewave performance and sonics.

Sayonara
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Old 26th January 2004, 07:23 PM   #18
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Kuei

Apologies if this is offtopic but i have recently developed a serious interest towards balanced PP amps as exemplified by Lynn Olson's Aurora, John Chapman's design, Marantz T1 and one or two of your own designs.

As building any of these is a serious undertaking i am still trying to get a handle on the expected sound. Lynn will have us believe it is actually better than SET and sliced bread at the same time, but based on my impressions of the Ariels i have a lingering doubt we may not share the same sonic preferences.

Have you actually built or listened to such a design? Using reasonble, but not sensational iron, as shown in the Aurora's schematics, can one really expect a wide subjective bandwidth? SE type midrange magic?
Interestingly the T1 uses parafeed after the first stage. Why?

Do you reckon the heroic effort of building one of these is really worth it or would a high-voltage high power SET be a better bet.

I'll probably buy the iron and do some tests anyway. Most likely with the mesh plate 300B from Valve Art as it seems reasonably priced.

Thanks.
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Old 26th January 2004, 07:38 PM   #19
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Quote:
suitable high nickel content core PP Interstage 1CT:1+1
What are we talking here? Sowter? S&B? other?

Stuff I consider highly without have real experience are Lynn Olson's ideas...

Valve rectification, (pseudo) choke input supplies, electrostatic screens on transformers. And also some stuff JamesD wrote over at K&K on how he has always tried to optimize each stage and isolate each stage as well as possible from the other (p (This is where an interstage transformer comes into the picture (hope I am understanding James well enough to explain it like this.)



Quote:
The importance of interstage isolation I learned from microwave techniques in receiver units when I worked at Decca Radar. I was tweaking existing designs for finer and deeper resolution and I got better results than the in-house experts by isolating each rf and if stage from each other physically and electrically. This beat the conventional wisdom of Faraday cage around each stage when in the field, although it worked less well in the open lab. This drove the in-house experts mad and taught me that absolute stage isolation and band pass control in and out was the ?secret? to achieving ?best? performance from any given stage. I guess it also made the point the rf is insidious stuff at any level and that ground isn?t ground! Later I found the same applied to high-end audio with a vengeance.

further I also "believe" in wooden chassis...solid core wiring...carbon resistors for signal applications.. star earthing...

Mmmm can't think of anything else..at the moment....

[edit]also posted just after analog_sa and am also interested on the "new" PP renaissance..Lynn, Gary Pimm and Allan Wright being some of the people (each in their own way) that I know of that are pioneering the PP topology...(forgive my ignorance)
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Old 26th January 2004, 08:08 PM   #20
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Default Shottky Diode?

Quote:
At least unil High Voltage schottky diodes that glow in the dark become cheaper than valve
Hi, Can you post a part number?



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